Multi beam scanning with bright/dark field imaging

ABSTRACT

Bright and dark field imaging operations in an optical inspection system occur along substantially the same optical path using the same light source by producing either a circular or an annular laser beam. Multiple beam splitting is achieved through the use of a diffractive optical element having uniform diffraction efficiency. A confocal arrangement for bright field and dark field imaging can be applied with multiple beam scanning for suppressing the signal from under-layers. A scan direction not perpendicular to the direction of movement of a target provides for improved die-to-die comparisons.

BACKGROUND

The invention relates to the field of optical inspection systems forinspecting semiconductor wafers, and more particularly to inspectingsemiconductor wafers using a scanned beam of laser light.

Although the title of this description indicates multi beam scanning, itwill be appreciated that the advances mentioned in some of theembodiments described below relate also to single beam scanning.

FIG. 1 shows a semiconductor wafer 10. Optical inspection systems areoften used to inspect dies 20 on the semiconductor wafer 10. FIG. 2shows several of the dies 20 of semiconductor wafer 10. Although it iscertainly possible, and very common to compare the pattern of conductorsof a die 20 with a reference image, it is also common for the comparisonto be a die-to-die comparison. That is to say, a die 20 is compared withanother die 20 instead of a reference image. For example, in adie-to-die comparison, dies adjacent to each other could be compared,such as the top to dies shown in the figure. Likewise, instead ofcomparing a die to another die in the same row, a comparison could bemade between one die and another in the same column.

FIG. 3 shows a beam of light 100 being made to impinge upon the surfaceof the die 20. The main optical scanning direction is indicated by theletter O. The mechanical scanning direction is indicated by the letterM. The mechanical scanning direction is the direction in which the stagemoves the wafer, and also may be referred to as the direction ofmovement of the wafer or the target movement direction.

FIG. 4 shows a system that employs dark field imaging. In FIG. 4, thewafer 10 is mounted on the X-Y stage 12. A light source 200 produceslight which is shaped, focused, or operated on as necessary by optics210 and provided to a scanner 300. The scanner 300 outputs the light insuch a manner that, after passing through optics 310, follows a scanpattern across the die 20 on the wafer 10. The scanner can beimplemented by a rotating polygon, deflection mirror, or acoustic-opticsdeflector (AOD). As is well known, dark field imaging uses detectors 410positioned so as to capture light 110 that is scattered rather thenreflected off of the surface of the die.

FIG. 5 shows a system that employs bright field imaging. In bright fieldimaging, the reflected light 120 is captured by a detector 420. Thedetector can be a PIN diode, PMT, or line CCD camera. The optics 310, inthis case, could include e.g. a beam splitter. Thus, light from thelight source 200 travels through beam shaper 205 and optics 210, and iscaused by the scanner 300 to impinge on the surface of the die 20 onwafer 10, and the reflected light 120 is channeled back through theoptics 310 to the bright field detector 420.

To put it another way, the bright field optical inspection systemcollects the specularly reflected light, whereas the dark field systemcollects the scattered light. Usually, a bright field system is usedwith very high-resolution imaging optics, and the inspection of thewafer is performed in such a manner that the pixel size is very small.The small pixel size makes maximum advantage of the high-resolutionimaging optics and the large amount of reflected light. Bright fieldsystems thus provide a great deal of detail, and are excellent when suchdetail is necessary.

Dark field systems provide a much higher throughput. Dark field systemstypically use laser scan technology for illumination, but the inspectionof the wafer is usually performed in such a manner that the pixel sizeis relatively large. The use of scattered light detection isadvantageous in that it has a high signal to noise ratio and evenrelatively small defects can be detected with high throughput. As willbe understood, higher throughput can be obtained with relatively lower,data rate.

It will also be appreciated that the systems of FIGS. 4 and 5 could becombined, resulting in a system having both bright field detectors anddark field detectors, positioned appropriately so that the scatteredlight can be detected by the dark field detectors and the reflectedlight can be detected by the bright field detectors as accomplished inApplied Materials wafer inspection tool, Compass™. It is important tonotice that in such a configuration the high throughput is obtained by ascanning spot with a relatively large pixel, so the resolution of the BFdetector is relatively poor.

What is needed is a better approach that provides higher throughput withalso high resolution in bright field mode.

SUMMARY OF THE INVENTION

In one aspect, the invention provides for an improved inspection methodand system in which a laser beam having a ring, or annular, shape isused.

In another aspect, the invention provides for an improved inspectionmethod and system for providing multiple beams.

In another aspect, the invention provides for an improved inspectionmethod and system in which a scan pattern is not perpendicular to thedirection of movement of the wafer.

In another aspect, the invention provides for an improved inspectionmethod and system in which a confocal arrangement is used.

In other aspects, various combinations of the foregoing features providefor improved inspection methods and systems having objects andadvantages that will be more fully appreciated after considering thenon-limiting exemplary embodiments presented below in the detaileddescription, taken together with the enclosed drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The enclosed drawing figures are for the sake of explanation only, andare not meant to be limiting to the scope of the invention. The drawingsare presented in highly simplified form. It will be appreciated that thedrawings freely omit elements which, although actually necessary for aconcrete realization of an actual apparatus, are well understood bythose familiar with this field; the omission of such routine elementsthus serves to focus attention on the key points in the description andavoids obscuring the description with unnecessary detail. The encloseddrawings are briefly described as follows:

FIG. 1 shows a semiconductor wafer.

FIG. 2 shows dies of the semiconductor wafer.

FIG. 3 shows a laser beam being scanned on a die.

FIG. 4 shows an inspection apparatus that uses dark field imaging.

FIG. 5 shows an inspection apparatus that uses bright field imaging.

FIG. 6A shows a pupil of an objective lens when a laser beam having acircular shape is used.

FIG. 6B shows a side view of the objective lens when the laser beamhaving the circular shape is used.

FIG. 7A shows a pupil of an objective lens when a laser beam having aring, or annular, shape is used according to an aspect of the invention.

FIG. 7B shows a side view of the objective lens when the laser beamhaving the annular shape is used.

FIG. 7C shows a side view of a conic prism pair suitable for producing alaser beam having an annular shape.

FIG. 7D shows the annular beam profile.

FIG. 7E shows the annular focused beam profile.

FIG. 8 shows an inspection apparatus according to an aspect of theinvention, in which a laser beam such as that shown in FIG. 6A is used.

FIG. 9 shows the inspection apparatus as in FIG. 8, in which a laserbeam such as that shown in FIG. 6B is used.

FIG. 10 shows a conventional inspection apparatus for inspecting a waferby using multiple beams.

FIG. 11 shows a conventional optical apparatus for producing multiplelaser beams from a single incident laser beam.

FIG. 12 shows an inspection apparatus for inspecting a wafer by usingmultiple beams produced according to an aspect of the invention.

FIG. 13 shows an optical apparatus for producing multiple laser beamsfrom a single incident laser beam, according to an aspect of theinvention.

FIG. 14 shows an optical apparatus as in FIG. 13, in which a laser beamis in a different position from that shown in FIG. 13.

FIG. 15 shows an exemplary CCD camera for collection, according to oneembodiment of the invention.

FIG. 16A shows a scan pattern for a laser beam in which the scan patternis perpendicular to the direction of movement of the wafer.

FIG. 16B shows an effective scan pattern corresponding to the scanpattern shown in FIG. 16A.

FIG. 16C shows how the effective scan pattern of FIG. 16B can berepresented by a die beam trace.

FIG. 16D shows, in beam trace form, three scans of the scan pattern ofFIG. 16A.

FIG. 17A shows a scan pattern for a laser beam in which the scan patternis not perpendicular to the direction of movement of the wafer,according to an aspect of the invention.

FIG. 17B shows, in beam trace form, three scans of the scan pattern ofFIG. 18A, according to an embodiment of the invention.

FIG. 18 shows, in beam trace form, ten scans of a multi beam scanpattern in which the scan pattern is not perpendicular to the directionof movement of the wafer, according to an aspect of the invention.

FIG. 19 shows an optical inspection system in which a confocalarrangement is used, according to an aspect of the invention.

DETAILED DESCRIPTION

Various embodiments of the aspects of the invention will now bepresented, and contrasted with conventional approaches. It will beappreciated that the embodiments are merely exemplary, and that many andvarious modifications can be made within the spirit and scope of theinvention. The scope of the invention is thus not restricted to theseexemplary embodiments, but is to be determined based solely on theappended claims.

FIG. 6A shows the top view of an objective lens pupil. Such an objectivelens might be used to focus a beam of light onto a die of a wafer 10.The entire pupil is shaded, and indicated by reference numeral 130. Thesignificance of this is to indicate that an incident beam of light is acircular laser beam (this will be contrasted with the beam shown in FIG.7A).

FIG. 6B shows the side view of the objective lens pupil shown in FIG.6A. The circular laser beam 130 is focused by the objective lens 320 toa desired target point on the wafer 10. The light that reflects off thetarget of the wafer 10 is all contained within the area of the pupilinto which the incident light was introduced (i.e., the angle ofincidence must equal the angle of reflectance for a reflected particleof light) In this case, the area of the pupil into which the is incidentlight was introduced happens to be the entire area. The scattered lightis all of the light 110 that is not reflected back into the objectivelens 320.

FIG. 7A shows the top view of the same objective lens pupil. Not all ofthe entire pupil is shaded, but only an annular part 150. An inner part160 is not shaded. The significance of this is to indicate that theincident beam of light 140 have been reshaped to have an annularprofile. This can be implemented by various optical means known to thoseskilled in the art of optics. For example it is possible to introduce anopaque filter in one of the beam pupil along the optical path to blockthe central part of the beam. Alternatively, it can be achieved by usingconic prism pair, as shown in FIG. 7C. To put it another way, theannular laser beam 140 has an annular part 150 that includes incidentlaser light, and an inner part 160 that does not include incident laserlight.

FIG. 7B shows the side view of the objective lens pupil shown in FIG.7A. The annular laser beam 140 is focused by the objective lens 320 to adesired target point on the wafer 10. The light that reflects off thetarget of the wafer 10 is all contained within the area of the pupilinto which the incident light was introduced. In this case, the area ofthe pupil into which the incident light was introduced is not the entirearea, but is only the annular part 150. Thus, the reflected lightreflects off the target and backup through the objective lens only inpart of the objective lens illuminated by the annular part 150 of theannular laser beam 140. The scattered light is all of the light that isnot reflected, and includes not only the part of the scattered light 110that is scattered away from the objective lens, but also the part of thescattered light 110 that is scattered up into the part of the objectivelens corresponding to the inner part 160 of the annular beam 140.

By virtue of using the annular beam 140, a very significant part of thescattered light 110 can be detected by collecting the scattered lightthat is scattered up into the part of the objective lens 320corresponding to the inner part 160 of the annular beam. The annularillumination profile (see FIG. 7D) gives high-resolution since thefocused annular beam size is comparable to that obtained with a fullilluminated pupil, as can be seen in FIG. 7E. The additional side lobecan be minimized by optimizing the radii ratio of the illuminated area.Using that, high resolution can be obtained even in dark field imagingoperations.

In FIG. 8, a light source 250 and optics 260 are capable of producingeither a circular beam 130 or an annular beam 140. For bright fieldimaging operations, the circular beam 130 is produced. For dark fieldimaging operations, the annular beam 140 is produced.

In FIG. 8, for example, bright field imaging operations are underway. Acircular beam 130 is being produced by the light source 250 and optics260. The beam is scanned by the scanner 300. The beam passes through thebeam splitter 330 and through the objective lens 320. Reflected light120 reflects back up through the entire area of the pupil of theobjective lens 320 and is reflected by the beam splitter 330 to beimaged by the detector 430.

In FIG. 9, for example, dark field imaging operations are underway. Anannular beam 140 is being produced by the light source 250 and optics260. The beam is scanned by the scanner 300. The beam passes through thebeam splitter 330 and through the objective lens 320. Scattered light110 passes, in large measure, through the part of the pupilcorresponding to the inner area 160 of the annular beam 140. Thescattered light 110 is reflected off the beam splitter 330 to be imagedby the same detector 430.

Because the size of the central part of the beam, through which thescattered light passes, is known it is of course possible to processjust this part of the light that is reflected from the beam splitter tothe detector 430 and to ignore the reflected light that is passing backthrough the outer part (i.e., the part corresponding to the annular part150).

Because the size of the central part of the beam and the outer part ofthe beam are both known, however, it is possible to process just thepart of the light returned in the inner part as dark field detection,and just the part of the light passed back through the outer part asbright field detection. Thus, when an annular beam is used, dark fielddetection and also bright field detection can simultaneously beperformed.

The imaging system according to this aspect of the invention includes anillumination system that selectively produces either a circular (usuallyGaussian) beam or an annular beam in response to a selection of imagingoperations type. That is, if only bright field imaging is selected, theillumination system is controlled to produce a circular beam 130; ifdark field imaging is selected, the light source is controlled toproduce an annular beam 140. Finally, it will also be appreciated thatthe circular beam 130 and the annular beam 140 are scanned and split inan identical manner.

While describing the foregoing aspect of the invention, in which anannular beam 140 is advantageously employed, only a single beam has beendiscussed. As will be recognized by one familiar with this field, thisapproach is also applicable in a multi-beam system such as the systemdepicted in FIG. 12 and described more fully below.

Inspection systems are known, in which multiple beams are created. Onesuch system, shown in FIG. 10, is described in U.S. Pat. No. 4,797,696to Allen, issued on Jan. 10, 1989. In such conventional systems as shownin FIG. 10, a light source provides a beam of light. The light is splitby a multiple beam splitter and then provided to the scanner. Thescanner scans the already-split beams of light through the optics ontothe wafer.

FIG. 11 shows a conventional example of how the multiple beam splitteritself might be implemented in a concrete sense. The illustration isalso taken from U.S. Pat. No. 4,797,696 to Allen. This patent isincorporated herein by reference for its useful background informationand concrete examples relating to optical inspection systems. In FIG.11, a single beam 501 is caused to impinge upon multiple beam splittingmembers 505. The beam is progressively split into multiple beams 170.The multiple beams are then scanned, but the scanning of multiple beamsis somewhat more complicated than scanning a single beam.

FIG. 12 shows an aspect of the invention which provides for simplifiedsplitting of beams and simplified scanning in a multiple beam system. InFIG. 12, a light source 200 provides a beam of light. The light isprovided to the scanner 300 via optics 210. After the scanner, the lightis provided to the multiple beam splitter 550 according to an aspect ofthe invention. The multiple beams are then caused to impinge on thetarget area of the wafer 10 via optics 310, and the resultant light iscollected as desired by bright field detector 420 or dark fielddetectors, depending on the type of imaging operation and imagingapparatus being involved. Of course, it is still possible, within thescope of the invention, to use the multiple beam splitter 550 and thenscan the multiple beams.

FIG. 13 shows the multiple beam splitter 550 in more detail. Inparticular, a single beam 501 is scanned left and right across theDammann grating 560.

Dammann gratings are well-known to those familiar with optics, althoughthey have never to the knowledge of the inventor been used in opticalinspection systems in the manner being described now. For furtherbackground reading, with the reader may refer to the following twoarticles: H. Dammann and K. Gortler, “High Efficiency in Line MultipleImaging by Means of Multiple Phase Holograms”, Optics Communication,Vol. 3, May, 1971; and “H. Dammann and E. Koltz, “Coherent OpticalGeneration and Inspection of Two-dimensional Periodic Structures”,Optica Acta, Vol. 24, 1977. These two documents are incorporated hereinby reference for their useful background information on Dammanngratings.

A Dammann grating can be computer designed and generated as desired bythe engineer to produce a plurality of beams (“Beam Brush”) from anincident beam, each of the plurality of beams having substantiallyidentical intensity (although not identity of direction). The pluralityof beams can be split from the incident beam in a desired plane.

In FIG. 13, the multiple beams 170 are produced by the effect of theDammann grating 560 on the incident beam 501. The multiple beams 170 arereceived by the relay 570 and then focused by the objective onto thesame number of scanning spots.

FIG. 14 shows a subsequent position of the beam 501. In particular, thebeam 501 has a different incident angle on the Dammann grating 560 andnow the multiple beams 170 are still produced, but also have a differentoutgoing angle perpendicular to the split axis. Still, the multiplebeams 170 are in the same orientation with respect to each other eventhough they have been tilted laterally. In FIG. 14, β indicates the scanangle. The scan direction is parallel to the Grating lines.

It will be appreciated that the system shown in FIGS. 12-14 provides foran optical inspection system and method in which the multiple beamsplitter is downstream from the scanner. Moreover, according to thisaspect of the invention, the multiple beam splitter includes a Dammanngrating for producing multiple beams 170 from a single incident beam501. In FIGS. 13 and 14, the incident beam 501 scans horizontally. Themultiple beams are produced in a vertical line. Thus, the systemaccording to this aspect of the invention provides for the production ofmultiple beams in a line perpendicular to the scanning direction of thelight output from the scanner. The result is a highly simplified systemsuch that the split is done after the scanning.

The Dammann grating is used here for the sake of example only, and itwill be appreciated that the Dammann grating can be generalized intoother structures not identical to those strictly adhering to thedescription in the two above-identified articles (much development hasoccurred since the 1970's), which may collectively be referred to asdiffractive optical elements having uniform diffraction efficiency(i.e., elements such as gratings that can split the incoming beam into nbeams of substantially identical intensity).

FIGS. 8 & 9 show basic optical schemes for the collection of the light.The scanning beam or beams may be imaged onto a line CCD or a multi lineCCD. Likewise, the beam or beams may each be focused into a PIN diode orPMT detector.

In FIG. 15, an exemplary CCD camera has rectangular pixels with arectangular pitch. The pitch size in the vertical (optical scan)direction is m*P where m is the required magnification and P is the sizeof a pixel. The pitch in the horizontal direction (the mechanicalscanning direction in the foregoing examples) is m*D where D is thedistance in between brush lines. The detector cell array should have afill factor as high as possible in the vertical direction, but much lessis required in the horizontal direction. An aspect ratio of 1:5 to 1:10would relax the alignment tolerances of the CCD with respect to theoptical scan direction. The number of pixels k and the number of lines nmay be, for example, 2000 pixels and 35 lines, respectively. It will beappreciated that other values may naturally be used in accordance withthe above principles and the particulars of a given situation. Usingthis type of camera allows for a detector in which each of the severalbeams is received on separate lines of CCD camera.

Another aspect of the invention will now be described with reference tothe FIGS. 16-19.

FIG. 16A shows a scan pattern for a laser beam in which the scan patternis perpendicular to the direction of movement of the wafer. Inparticular, the beam spot 100 is scanned by a scanner along a lineindicated by 600. The line indicated by 600 may be thought of as ascanning direction or a scan pattern. The scan pattern shown in 16A is apattern represented as being independent of the wafer on which it isscanned. Another way to think of this, is that the scan pattern is shownas if the wafer were not being moved. In actuality, the wafer isconstantly being moved.

FIG. 16B is an illustration that takes into account the movement of thewafer. It is well understood that the wafer is moved by the stage 12 sothat the entire surface of the wafer can be inspected. In this figure,601 represents the initial position of the beam spot 100 at thebeginning of the scan. As a small amount of time progresses, the waferis moved slightly in the mechanical scanning direction M and the beamspot 100 moves in the optical scanning direction 0 to the positionindicated by 602. As the wafer is moved even more, the beam spot 100 ismoved through positions 603 and 604 to the end of the scan pattern atposition 605. After this, the beam spot is instantaneously moved to theposition indicated by 601′.

It will be appreciated that positions 601 through 605 are not discretepositions, but the beam is moved smoothly and continuously between thebeginning scan position 601 and the final scan position 605. When thebeam is moved from the final scan position 605 to the initial scanposition 601′ for the next scan cycle, it is moved discontinuously, and“skips” back to the initial scan position.

In FIG. 16B, the distance indicated by T represents the distance inbetween scan lines along the stage movement direction (i.e., in themechanical scanning correction). That is to say, T represents how farthe stage is moved during the time it takes for the scanning beam tomove from the initial scanning position 601 of a present scan cycle tothe initial scanning position 601′ of the next scan cycle.

The illustrations shown in FIGS. 16A and 16B are two ways ofrepresenting the same physical result. FIG. 16A may be said to show ascan pattern, while FIG. 16B may be said to show an effective scanpattern. Thus, the effective scan pattern is a representation that takesinto account the movement of the wafer in the mechanical scanningdirection.

Another way to represent the effective scan pattern is shown in FIG.16C. Here, a solid line 610 shows the movement of the center of the beamspot starting at the initial scan position and moving through the finalscan position. A line of dashes 615 between the final scan position andthe initial scan position of the next scan cycle represents the jump ofthe beam spot to move into position for the next scan cycle. The line610 may be referred to as the effective scan line, and the line 615 maybe referred to as the effective return scan line.

FIG. 16D shows, in beam trace form, three scans of the scan pattern ofFIG. 16A. Although the scan pattern shown in FIG. 16A has a scandirection 600 that is perpendicular to the mechanical scanning directionM, it is apparent that the effective scan pattern is such that theinitial position for the next scan cycle 601′ is below the final scanposition 605 of the previous scan cycle. In other words, the distance,in the mechanical scanning direction M, traveled by the beam during thescan cycle is substantially equal to T (how far the stage is movedduring the time it takes for the scanning beam to move from the initialscanning position 601 of a present scan cycle to the initial scanningposition 601′ of the next scan cycle).

FIG. 17A shows a scan pattern for a laser beam in which the scan patternis not perpendicular to the direction of movement of the wafer,according to an aspect of the invention. In particular, the scan patternfor the beam spot 100 includes a scan direction 650.

In FIG. 17B, M represents the direction of movement of the wafer (i.e.,the mechanical scanning direction). D represents the distance in betweenthe brush lines. T represents the distance in between scan lines in themechanical scanning direction. FIG. 17B shows, in beam trace form, threescans of the scan pattern of FIG. 17A, with 660 representing theeffective scan line and 665 representing the scan return scan line.Because the scan direction 650 is not perpendicular to the mechanicalscanning direction, the beam spot travels a distance in the mechanicalscanning direction that is always greater than T. In both FIGS. 17 b and18, M′ represents a reference line that is in the same direction as themechanical scanning direction, and is shown to make clear the existenceof an angle α. This angle α may be referred to as the scan line tiltangle. P represents the distance between scan lines, and thus the pixelsize.

FIG. 18 shows, in beam trace form, ten scans of a multi beam scanpattern with a beam brush of n=4 in which the scan pattern is notperpendicular to the direction of movement of the wafer. In FIG. 18, Mrepresents the direction of movement of the wafer (i.e., the mechanicalscanning correction). D represents the distance in between the brushlines. T represents the distance in between scan lines in the mechanicalscanning direction. HFOV represents the number of pixels in thehorizontal field of view. NPL represents the number of pixels along thescan line.

Because the beam spot travels a distance in the mechanical scanningdirection that is greater than T, a very accurate die-to-die comparisoncan be obtained, with increased assurance that the same line has scannedthe same place on the two different dies being compared. Of course, itgoes without saying that the comparison between the two different diesmust be made at different times in order for the same line to scan thesame place on the two different dies. The data can be buffered in amanner well-known to those familiar with this field, and stored in amemory as necessary until the comparison can be made.

From the foregoing, and as can be seen from FIG. 18, the relations shownbelow follow directly (assuming that τ is the amount of time it takesfor the stage to move the distance T) and can be used to determine thestage velocity and data rate: $\begin{matrix}{D = {{\frac{HFOV}{n - 1}\quad\ldots\quad{where}\quad n} > 1}} & ( {{Equation}\quad 1} ) \\{{\sin\quad\alpha} = {\frac{D}{NPL} = \frac{HFOV}{( {n - 1} ) \cdot {NPL}}}} & ( {{Equation}\quad 2} ) \\{T = {\frac{P}{\sin\quad\alpha} = {\frac{P}{( \frac{HFOV}{( {n - 1} ) \cdot {NPL}} )} = \frac{P \cdot ( {n - 1} ) \cdot {NPL}}{HFOV}}}} & ( {{Equation}\quad 3} ) \\{{DR} = \frac{n \cdot {NPL}}{\tau}} & ( {{Equation}\quad 4} )\end{matrix}$where

-   n=number of beams-   P=pixel size-   HFOV=number of pixels in horizontal field of view-   NPL=number of pixels along the scan line-   DR=data rate in pixels per second-   D=distance in between brush lines-   T=distance in between scan lines in the stage movement direction-   τ=the scan line time-   α=the scan line tile angle.

FIG. 19 shows another aspect of the invention, in which in which aconfocal arrangement is used for optical focus.

In FIG. 19, a light source 900 provides light through a beam shaper 905.The beam shaper provides the light through the pupil 907 and themultiple beam splitter 908 (which may be a Dammann grating). Themultiple beams are provided to a beam splitter 910 and then throughscanning means 915, 920. The scanner outputs light to a relay 925 and abeam splitter 930. The beam splitter 930 illuminates a target on wafer945 through first objective lens 940 using magnification telescope 935.

Light returned back from the illuminated spot on the specimen, collectedby the objective 940, passes back through the scan unit in order tocancel the optical scanning, so that the returning beams will be staticupon the detectors. Then the reflected beams, which may be thought of asa returned light signal, are deflected by beam splitter 910 towards afocusing lens 912. The lens 912 focuses all the beams to a pinhole 913in order to obtain the confocal effect. After passing the pinhole eachbeam is collected (by means of additional optical means, not shown, butreadily implemented by one familiar with this field) onto a respectiveindependent detector 914. This confocal pinhole 913 (which may bereferred to as a confocal optical element) is what gives the system itsconfocal property, by rejecting light that did not originate from thefocal plane of the microscope objective. Light rays from below the focalplane come to a focus before reaching the detector pinhole, and thenthey expand out so that most of the rays are physically blocked fromreaching the detector by the detector pinhole.

In the same way, light reflected from above the focal plane focusesbehind the detector pinhole, so that most of that light also hits theedges of the pinhole and is prevented from reaching the detector 914.However, all the light from the focal plane is focused at the detectorpinhole in blocking member 913 and so is detected at the detector 914.

Because of the confocal arrangement, the depth of focus of theinspection apparatus is greatly narrowed, making it possible to suppressthe signals from the under-layers. The signals from the under-layers inmost cases are not important and introduce noise to the image processingsystem. Also it is possible to use the confocal arrangement togetherwith annular illumination mode in order to suppress DF signals fromunder layers.

For the sake of completeness, it will be mentioned that the imaginglight reflected from the wafer 945 passes up through the beam splitter930 and through imaging lens 950 to another beam splitter 955. In thisexemplary arrangement, the bright field channel light is collected byCCD camera 960. The dark field channel light is collected by a CCD orPMT 965.

In another aspect of the invention, an optical inspection system mayinclude various combinations of the foregoing other aspects. Forexample, the use of an annular beam for dark field / bright fieldoperations may be combined with a multiple beam brush created using adiffractive optical element having uniform diffraction efficiency, andmay be combined with a system in which the scan direction is notperpendicular to the mechanical scanning direction, and may be combinedwith a system having a confocal focus arrangement. Likewise, thecreation of multiple beams using a diffractive optical element havinguniform diffraction efficiency may be combined with a system in whichthe scan direction is not perpendicular to the mechanical scanningdirection, and may be combined with a system having a confocal focusarrangement. Likewise, a system in which the scan direction is notperpendicular to the mechanical scanning direction may be combined witha system having a confocal focus arrangement. Finally, combinations ofany or all of the aspects of the invention are also possible.

As mentioned above, the exemplary embodiments shown in the figures anddescribed above are for the sake of explanation and are not meant to belimiting upon the invention. Many specificities have been mentioned, butthese are also not meant to be limiting on the invention. Rather, thescope of the invention is to be interpreted in accordance with thespirit of the invention, in accordance with the claims below.

1. An optical inspection method, comprising: outputting an annular beamfrom a light source; focusing the annular beam at a target; anddetecting light scattered from the target; wherein: the detecting of thescattered light detects the light scattered through a portion of anobjective lens corresponding to an inner part of the annular beam; andsimultaneously with the detecting of the scattered light there is adetection of light reflected from the target, as bright field detection,through a portion of the objective lens corresponding to an outer partof the annular beam.
 2. An optical inspection method, comprising:outputting an annular beam from a light source; scanning the annularbeam along a line in a given scanning direction to provide a scannedsingle annular beam; producing multiple annular beams of substantiallyidentical intensity from the scanned single annular beam; focusing themultiple annular beams at a target; and detecting light scattered fromthe target; wherein: the detecting of the scattered light detects thelight scattered through a portion of an objective lens corrosponding toan inner part of each of the annular beams; and simultaneously with thedetecting of the scattered light there is a detection of light reflectedfrom the target, as bright field detection, through a portion of theobjective lens corresponding to an outer part of each of the annularbeams.
 3. An optical inspection method, comprising: outputting anannular beam from a light source; scanning tho annular beam along a linein a given scanning direction to provide a scanned single annular beam;producing multiple annular beams of substantially identical intensityfrom the scanned single annular beam: focusing the multiple annularbeams at a target; and detecting light scattered from the target;wherein the detecting is performed with a multiple line CCD camera, andincludes imaging each of the multiple annular beams on a separate one ofthe lines of the multiple line CCD camera.
 4. An optical inspectionmethod comprising: outputting a single beam; scanning the single beamalong a line in a given scanning direction to provide a scanned singlebeam; and producing multiple beams of substantially identical intensityfrom the scanned single beam; wherein: the producing of the multiplebeams is performed with a diffractive optical element having uniformdiffraction efficiency; and the diffractive optical element is a Dammanngrating.
 5. An optical inspection method, comprising: outputting asingle beam; scanning the single beam along a line in a given scanningdirection to provide a scanned single beam; producing multiple beams ofsubstantially identical intensity from the scanned single beam; focusingthe multiple beams at a target through an objective lens; receivinglight returned from the target, through the ohjective lens; anddetecting the returned light with a multiple line CCD camera by imagingeach of the multiple annular beams on a separate one of the lines of themultiple line CCD camera.
 6. An optical inspection method, comprising:providing a beam of light; providing scanned multiple beams from thebeam of light; illuminating a target, with the scanned multiple beams,through an objective lens; collecting light, returned back from theilluminated target with the objective lens; passing the collected lightthrough to an imaging lens; and focusing the light of the imaging lensto a bright field channel detector; wherein the bright field channeldetector includes a multiple line CCD camera, and wherein each of themultiple annular beams is received on a separate one of the lines of themultiple line CCD camera.
 7. An optical inspection method, comprising:providing a beam of light; providing scanned multiple beams from thebeam of light; illuminating a target, with the scanned multiple beams,through an objective lens; collecting light, returned back from theilluminated target, with the objective lens; passing the collected lightthrough to an imaging lens; and focusing the light of the imaging lensto a bright field channel detector; providing a beam splitter opticallydisposed between the imaging lens and the bright field channel detector;and focusing a portion of the light, from the imaging lens, through thebeam splitter and also on a dark field channel detector.
 8. The opticalinspection method as set forth in claim 7, wherein at least one of thebright field channel detector and the dark field channel detectorincludes a multiple line CCD camera and wherein each of the multipleannular beams is received on a separate one of the lines of the multipleline CCD camera.
 9. An optical inspection method, comprising: providinga beam of light, from a light source, through a pupil; receiving thelight through the pupil at a multiple beam splitter; splitting the beamof light using the multiple beam splitter; providing the scanned lightas multiple scanned beams; passing the resulting light through a firstbeam splitter; scanning the light received through a first beam splitterto provide scanned light; passing the scanned light through a scan lensand a second beam splitter, and illuminating a target through anobjective lens; collecting light returned back from the illuminatedtarget; passing the collected light to the second beam splitter.providing part of the collected light, from the second beam splitter,through an imaging lens to a bright field channel detector; providingpart of the collected light, as a returned light signal, back throughthe scan lens and scanner to the first beam splitter; deflecting thereturned light signal, with the first beam splitter, through a focusinglens and a pinhole; and receiving the light through the pinhole usingone or more detectors.
 10. The optical inspection method as set forth inclaim 9, wherein the bright field channel detector includes a multipleline CCD camera, and wherein each of the multiple annular beams isreceived on a separate one of the lines of the multiple line CCD camera.11. The optical inspection method as set forth in claim 9, furthercomprising: providing a third beam splitter optically disposed betweenthe imaging lens and the bright field channel detector; and focusing thelight from the imaging lens through the third beam splitter also onto adark field channel detector.
 12. The optical inspection method as setforth in claim 11, wherein the multiple scanned beams are provided asannular beams.
 13. The optical inspection method as set forth in claim11, wherein at least one of the bright field channel detector and thedark field channel detector includes a multiple line CCD camera, andwherein each of the multiple annular beams is received on a separate oneof the lines of the multiple line CCD camera.
 14. The optical inspectionmethod as set forth in claim 9, wherein the multiple beam splitter is adiffractive optical element having uniform diffraction efficiency. 15.The optical inspection method as set forth in claim 14, wherein thediffractive optical element is a Dammann grating.